Effects of catecholamines on inotropism, chonotropism and relaxation of heart muscle are thought to be associated with stimulation of heart adenylyl cyclase resulting from interaction of the amine with beta-receptor. If used for prolonged time and at high concentrations, catecholamines also induce refractoriness to their own stimulation. Induction of refractoriness appears to be associated with desensitization of adenylyl cyclase to amine stimulation, possibly via uncoupling and loss of beta-receptor. The long term goal of this research is to elucidate the primary molecular mechanisms involved in the actions of beta-receptors on heart tissue by investigating mechanisms of stimulation of adenylyl cyclase and by determining quantitative relationships between beta-receptor occupancy, beta-receptor abundancy, adenylyl cyclase stimulation, adenylyl cyclase desensitization, and inotropic and chronotropic effects of catecholamines. Specific aims for the proposed period include: 1) studies on the characteristics of particulate rat heart adenylyl cyclase; 2) studies on conditions leading to desensitization in tissue fragment evaluating results by determination of adenylyl cyclase and inotropic and chronotropic properties of the tissue; 3) comparative exploration of receptor characteristics for stimulatory and desensitizing actions of catecholamines; 4) development of a radioligand receptor assay for mammalian heart beta-receptor using iodohydroxybenzyl pindolol; 5) exploration of modes of coupling of beta-receptor to adenylyl cyclase; 6) comparative study of control and desensitized adenylyl cyclase; 7) determination of receptor abundancy or availability in desensitized tissue; 8) exploration of conditions leading to reversal of desensitization and of refractoriness in isolated organ studies; 9) studies on ways to obtain desensitization in membrane particles. These studies, which use specific binding and desensitization of adenylyl cyclase as tools, should give insight into receptor-contraction coupling (spare receptors) in intact heart.